Dual energy CT utilises two separate energy sets to examine the differing attenuation properties of matter, having a significant advantage over traditional single energy CT. Independent attenuation values at two energy sets can create virtual non-contrast images from contrast enhanced imaging as well as delineate the composition of renal calculi and arterial plaque 13.
There are three different dual energy technologies available:
- dual source dual energy
- two x-ray tubes producing different voltages offset at 90 degrees
- single source dual energy
- a single x-ray tube with fast switching voltage otherwise known as kVp switching
- single source dual layer
X-ray photons primarily interact with matter via the photoelectric effect, and Compton scattering producing the diagnostic images used in medicine today.
When an atom undergoes the photoelectric effect, the electron from that respected K-shell otherwise referred to as the inner shell is ejected via the incident photon. As that electron is excited, vacant space is 'filled' by a neighbouring electron, releasing energy as a photoelectron.
In short, when a photon has sufficient energy to overcome the electron's binding energy in the K-shell, that atom undergoes the photoelectric effect.
Each substance owns a unique K-shell binding energy; known as the K-edge. There is a significant spike in attenuation that results just beyond the energy of the K-edge, this peak is unique to every material and holds valuable information about the substance's composition.
The different photoelectric energies and K-edges are the bread and butter of dual-energy CT. Although most elements in the human body have very low K-edges (0.01-0.53 keV) elements like iodine and calcium, have a K-edge of 33.2 keV and 4.0 Kev respectively, making them sufficiently larger than surrounding structures and especially important in the clinical setting 1-3.
For instance, at 80 kVp a structure that contains no (introduced) iodine i.e. the liver has an attenuation based on its K-edge of x, yet when iodine (33.2 keV) is introduced into that same structure, it has a higher attenuation of y bringing it closer to 80 kVp.
As 80 kVp is closer to 33.2 keV than 140 kVp, the structures containing iodine will retain less attenuation as the kVp progressed beyond the K-edge of iodine. Therefore, when using two energies, it is possible to delineate structures based solely on their attenuation differences between 80 kVp and 140 kVp.
A dual x-ray source, tube A (140 kVp) and tube B (80 kVp or 100 kVp) with an angular offset of 90 degrees are preferred offsets for a dual source scanner in the current literature 1-5.
- 1. Vlahos I, Godoy MC, Naidich DP. Dual-energy computed tomography imaging of the aorta. J Thorac Imaging. 2010;25 (4): 289-300. doi:10.1097/RTI.0b013e3181dc2b4c - Pubmed citation
- 2. Godoy MC, Heller SL, Naidich DP et-al. Dual-energy MDCT: comparison of pulmonary artery enhancement on dedicated CT pulmonary angiography, routine and low contrast volume studies. Eur J Radiol. 2011;79 (2): e11-7. doi:10.1016/j.ejrad.2009.12.030 - Pubmed citation
- 3. Lu GM, Wu SY, Yeh BM et-al. Dual-energy computed tomography in pulmonary embolism. Br J Radiol. 2010;83 (992): 707-18. doi:10.1259/bjr/16337436 - Free text at pubmed - Pubmed citation
- 4. Ascenti G, Mazziotti S, Lamberto S, Bottari A, Caloggero S, Racchiusa S, Mileto A, Scribano E. Dual-energy CT for detection of endoleaks after endovascular abdominal aneurysm repair: usefulness of colored iodine overlay. Am J Roentgenol. 2011 Jun;196(6):1408-14. doi: 10.2214/AJR.10.4505.
- 5. Johnson TR. Dual-energy CT: general principles. AJR Am J Roentgenol. 2012;199 (5_supplement): S3-8. doi:10.2214/AJR.12.9116 - Pubmed citation
- 6. Rutherford RA, Pullan BR, Isherwood I. Measurement of effective atomic number and electron density using an EMI scanner. Neuroradiology. 1976;11 (1): 15-21. Pubmed citation
- 7. Coursey CA, Nelson RC, Boll DT et-al. Dual-energy multidetector CT: how does it work, what can it tell us, and when can we use it in abdominopelvic imaging?. Radiographics. 2010;30 (4): 1037-55. doi:10.1148/rg.304095175 - Pubmed citation
- 8. Reagan AC, Mallinson PI, O'Connell T et-al. Dual-energy computed tomographic virtual noncalcium algorithm for detection of bone marrow edema in acute fractures: early experiences. J Comput Assist Tomogr. 2014;38 (5): 802-5. doi:10.1097/RCT.0000000000000107 - Pubmed citation
- 9. Pache G, Krauss B, Strohm P et-al. Dual-energy CT virtual noncalcium technique: detecting posttraumatic bone marrow lesions-feasibility study. Radiology. 2010;256 (2): 617-24. doi:10.1148/radiol.10091230 - Pubmed citation
- 10. Guggenberger R, Gnannt R, Hodler J et-al. Diagnostic performance of dual-energy CT for the detection of traumatic bone marrow lesions in the ankle: comparison with MR imaging. Radiology. 2012;264 (1): 164-73. doi:10.1148/radiol.12112217 - Pubmed citation
- 11. Stolzmann P, Scheffel H, Rentsch K et-al. Dual-energy computed tomography for the differentiation of uric acid stones: ex vivo performance evaluation. Urol. Res. 2008;36 (3-4): 133-8. doi:10.1007/s00240-008-0140-x - Pubmed citation
- 12. Hidas G, Eliahou R, Duvdevani M et-al. Determination of renal stone composition with dual-energy CT: in vivo analysis and comparison with x-ray diffraction. Radiology. 2010;257 (2): 394-401. doi:10.1148/radiol.10100249 - Pubmed citation
- 13.Machida H, Tanaka I, Fukui R et-al. Dual-Energy Spectral CT: Various Clinical Vascular Applications. Radiographics. 2016;36 (4): 1215-32. doi:10.1148/rg.2016150185 - Pubmed citation